Current sensors are available, which are being used for a wide range of applications, such as being part of indicator systems indicating different electrical states in a conductor based thereupon, such as momentary and average current values, power values, the presence of short circuits, faults on the line, circuit breaker cut outs and the like. In order to provide reliable current sensing an accurate, real-time status feedback is needed. Current sensors are available for DC and AC electrical systems. In the context of the present application, only current sensors for conductors leading AC current are considered and discussed.
One example of an electrical state to be indicated is short circuits. The indicator system being discussed below relates to indicating short circuits. However, it has by the invention been realized that a wide range of current states or current based conditions may be monitored by a current sensor according to the invention, and an indicator system according to the invention comprising such current sensor may be used to indicate a multitude of electrical states based thereupon. The indicator system adapted for short circuit detection should thus not be construed as limiting the invention, but rather exemplifying it.
When short circuits occur during operation of electrical AC power lines, such as for example high and medium AC voltage distribution and supply power lines in a distribution network or power lines for rail roads, it is important to locate and repair these as quickly as possible. To that end, short circuit indicators are available on the market.
Generally, known types of short circuit indicators comprising a current sensor for electrical AC power distribution lines are available for remote or local indication of the presence of short circuits thereon in order to reduce localization time for maintenance work and the associated costs therewith. In particular when measuring high voltage AC power lines, the problem is to determine the occurrence of such short circuit, e.g. by measuring the current or more correctly the short circuit current, without a potential connection to the electric AC power line, which would be too costly due to the requirement of having a high degree of insulation between conductor and ground, e.g. a remotely positioned indicator unit.
The short circuit currents experienced in an electric conductor in AC power lines are often several orders higher than the normal state current, depending on conductor impedance and distance between the sensor and the position of the short circuit on the conductor. The value of the measured currents at which the indicators are set to indicate a short circuit, i.e. the short circuit current threshold, may be adjustable because the set threshold is dependent upon conditions at the application point, e.g. different cable cross sections and changing electrical load.
Some available types of short circuit indicators for AC conductors comprise one sensor for each phase or conductor being monitored, and comprise two different types:
A first sensor type provides the indicator of a short circuit adjacent to the sensor i.e. integrated with the sensor being mounted upon the monitored conductor in a sensor-indicator unit. That is an advantage when monitoring e.g. overhead power lines, which are freely visible from the ground, and the indicator is e.g. mechanical or a powerful light flashing or alternatively an RF signal being emitted at the moment of a short circuit being sensed. An example of such sensor-indicator units are described in e.g. U.S. Pat. No. 5,748,095 for local overhead display of a short circuit state by a flashing light or a red dot on the line being monitored, while during normal state currents, no light or a green dot is provided.
The second type provides the indication unit remotely from each sensor unit, e.g. one for each of the three phases being monitored, in a short circuit indicator system. That is an advantage when monitoring e.g. ground cables, which are not freely visible, and thus the indicator unit is placed in a position, which allows for easy maintenance thereof. An example of such short circuit indicator system is described in EP 0 463 860. Here also, the indication is provided when a fault or short circuit has been sensed by the sensor unit.
A conventional current sensor for the two indicator type comprise a current transformer, which may be an open or clamp-on type transformer consisting of an attachable magnetic material yoke for arranging around one conductor or power line forming the first winding, i.e. a single transformer turn in the transformer. The secondary transformer winding is wound around the magnetic material, e.g. with several windings for supplying the current sensor unit with a measurement current. This type of current transformer allows installation on power system cables without disconnecting the power system cables from their source or load.
Generally, the first type of available indicators comprising a current sensor is suitable for AC electrical power lines of all voltages, and the second type of available indicator is suitable for AC voltage due to the isolation required between sensor and the ground level provided at the indicator unit.
However, many of the short circuits experienced by the power distribution companies are provided in high, medium and low Amp, medium voltage (MV) systems, i.e. from 1 to 40 kV and low voltage (LV), i.e. 110 to 480 V AC power lines. Even if the frequency of experienced short circuits is low, some power distribution companies have imposed fines when power outages occur on their distribution network, which fact presents an incentive for these companies to increase up-time in all areas of the power grid. The indication of short circuits is also needed in related MV and LV fields, e.g. rail road operational power lines transmitting AC voltage around 1 kV, and distribution power lines around 400 V.
Accordingly, automated, remotely monitored indicators are needed, which are suitable both for HV, MV and LV power AC distribution cables and for a wide range of conductor current levels.
As an example, in an electric power distribution network, there is in particular in the medium voltage (MV) and low voltage (LV) grid a multitude of distribution nodes or stations, which delivers connections to different consumer groups. The majority of current carrying lines are at present provided as underground cables; only a few locations present air cables, especially in rural areas. By the presence of faults or specific current states such as short circuits in the grid it is important to locate and correct the errors quickly and reliably, and important to be able to differentiate between temporary current surges and other lasting specific current states, such as short circuits.
An advantage of remote monitoring of e.g. short circuits in the many MV and LV AC power lines is that identification of problematic power lines may be performed easily and quickly for a determination of the position and extent of investments in improved grid infrastructure, maintenance of older and/or failing equipment and replacement thereof. This considerably decreases maintenance and planning costs for the electric power distribution companies.
The above mentioned EP 0 463 860 disclose a faulted circuit indicator having a detector which indicates fault currents in a monitored conductor to the remotely located indicator using an electrically isolated signal link there between. The detector comprises a current sensor which is connected to a load carrying conductor, and the detector is detecting the presence of a fault or system disturbance in the monitored conductor and is signalling the indicator unit of such an event. When a fault of a predetermined current magnitude is detected by said detector, a light pulse is transmitted from the detector to the indicator via the link i.e. a fibre optic cable, where upon receipt the pulse is converted to an electrical pulse causing the indicator to indicate a “fault” state. The use of an electrically isolated fibre optic cable ensures galvanic separation between the power line and the indicator, and also reduces problems with electric interferences upon the relayed detection signal.
However, said known detector may not be sufficiently fast to both sense, detect and convey all instances of short circuits during the available detection time period before the line current has reached zero. Having a short circuit happening close to the indicator reduces the duration of the short circuit current experienced and increases the maximum current value thereof, because these two factors are dependent both upon the distance from the indicator position to the short circuit on the conductor and upon the normal state current in the power lines. Further, said detector may not be sufficiently precise to distinguish between rush-in current peaks and real short circuits resulting in blown fuses.
Thus, the prevalent technical prejudice has been that self-sufficient current sensors in the above mentioned two types of known indicator systems for HV power lines have been too slow to indicate nearby short circuits, in particular if attempted applied to LV and MV power lines. As an example, in a 1 kV rail road power line, the disconnection time period for the short circuit release is reduced to 5 ms when experiencing a nearby short circuit. Accordingly, the detection time period for providing a reliable current sensor sensing of a short circuit is around or shorter than 5 ms. This time period is equal to one fourth of a period in a 50 Hz system, which period is the shortest known measurement interval needed for prior art AC short circuit indicators.
WO 2004/099798 discloses a current measurement device, a system comprising such device and a compensation method for calibrating such system, wherein the device comprises a fibre optic Faraday effect current sensor for an electric current measurement in MV power lines. The sensor comprises a magneto-optical material being placed transversally to the current in a power line and measures the Faraday effect of the current therein, which states that the plane of a polarized incident light undergoes a rotation relative to the magnetic field applied. The sensors are connected to measuring modules using optical fibres, which guide the polarized light. As stated in the document a disadvantage of these Faraday effect sensors is that there are sources for disturbances of the detection of the polarization change of the light signal, comprising optical noise in the detection circuit, electrical noise in light source, interference from magnetic fields from nearby inductors and systems, sensor mounting and setup, conductor shape and diameter, sensor production tolerances, temperature effects on light source and detector, light source and detector degradation over the products lifetime. Thus, calibration both prior to commencing measurements and during monitoring the lines is necessary, which increases the costs of the sensor and indicator units alike. Given the fact that a large number of devices is required for enabling a network monitoring system, this increases the total cost of the disclosed system.
In U.S. Pat. No. 6,566,855 is described a device or sensor measuring current magnitude in a conductor coupled to an electrical device. Said device generates an electrical output signal comprising a series of pulses having a frequency, where said frequency of the series of pulses is responsive to the magnitude of current measured. The device is applied to sensing current for different types of electrical loads, such as pumps, compressors, heaters, conveyors and the like for LV and AC current conductors. However, said device is not suitable for monitoring HV or even MV conductors, because the device needs to be sufficiently electrically insulated between conductor and ground. This is due to the fact that the device utilises a low voltage optical isolator, and the device may only at a high cost be adapted to higher voltage applications. Further, the frequency signal is transmitted from the device to the digital controller via electrical cables. Accordingly, an application to HV and MV AC conductors would require the application of an expensive electrical isolation between device and conductor, or indicator and ground. Further, the sensor disclosed is too slow to provide time-realistic current values as it provides an average current value measured over a relatively long sampling time period, i.e. more than one cycle period, as may be seen from the circuit diagram provided.
Accordingly, it is an object of the present invention to provide a low cost AC current sensor suitable for integration in an indicator system, e.g. for indicating short circuits, and corresponding indication system, which provides for rapid detection of current states in electric AC conductors, in particular being easily adaptable for use in detecting a wide range of conductor current magnitudes in both LV, MV and HV power distribution, e.g. for detecting short circuits. Further, an object of the present invention is that the current sensor, and thus also the indicator system must be able to provide reliable indications of such current states, such as short circuits within short detection time periods, e.g. shorter than 5 ms.